Metal vapor discharge lamp



Sept. 7, 1943. E, LEMME 2,329,125

METAL VAPOR DISCHARGE LAMP Filed Dec. 50', 1940 .lnven'tor Euene Lemmer's,

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Patented Sept. 7, 1943 'UNITEDYVSTATES PATENT OFFICE: 2, 29,125

METAL VAPOR DISCHARGE LAMP Eugene Lemmers, Cleveland Heights, Ohio, assig'nor to General Electric Company, a corporation of New York Application December :0, 1940, Serial No. 372,274

Claims. (cl. 176-124) called capillary lamps of the type shown for example in the United States Patent No. 2,094,694 to Cornelis Bol are examples of this practice. Under the conditions of operation of these lamps, the discharge emits light having broad spectral bands in place of the sharp hnes characteristic oi the spectra of most low pressure gaseous or vapor discharges. In addition, a background of radiation which is probably the result of thermal activity and which has a continuous spectrum becomes apparent. Under sumciently high pressures, the broad bands and the continuous background radiation taken together give a spectrum approaching that of white light. The obvious dsadvantage of such arrangements is that pressures are involved which are so high that in one known case a water pressure of 10,000 pounds per square inch outside the lamp was necessary to keep the lamp from exploding. This not only entails considerable danger during operation but also adds greatly to the cost of the lamps.

It is my belief that the principal effect of the high pressure is to constrict the arc to a narrow thread in the center of the discharge chamber and thus to raise the current density within the arc stream itself. It is my further belief that it is the very high current density which causes the arc to emit light having broad spectral bands.

Now I have found that it is not necessary to use excessive pressures to raise the current density to the necessary values and that the same results can be accomplished by constricting the arc with the chamber walls themselves rather than by means of the pressure of the gaseous atmosphere, i. e. by using a chamber having a crosssection small enough to give rise to" the required density even when the arc fills the entire chamber as it tends to do at low pressures. With such an arc chamber, the vapor pressure can be maintained at a safe value without adversely affecting the quality of the emitted light. In fact, I have found that the spectral quality can be changed for the better for reasons to be explained presently. Ifthe arc current densities are increased far beyond those generallyemployed in the presently available capillary lamps, it is possible, I have found, to produce a. number oi. the lines of the spark spectrum of the vapor used to carry the arc. The addition of these to the combined broad band and continuous spectrum already discussed causes the resultant spectrum of the lamp to' still further approach that of pure white light. Current densities great enough to produce these spark spectrum lines can only be obtained in the conventional capillary lamps at pressures far beyond the range of practicability. They may, however, be obtained with comparative ease by the methods of my invention.

Accordingly, it is an object of my invention to provide relatively low pressure vapor discharge lamps for producing light of the quality normally produced only by the known high pressure metallic vapor lamps.

It is another object of my invention to provide a discharge lamp of the aforesaid type which will produce light of an improved spectral quality.

It is still another object of my invention to provide a new and improved lamp of the aforesaid type which shall be simple in construction and more economical in manufacture and operation.

Eurther objects and advantages or my invention will become apparent from the following detailed description and accompanying drawing the single figure of which shows a lamp constructed in accordance with the invention,

together with a suitable operating circuit.

Referring to the drawing, the lamp i comprises an envelope 2, preferably of quartz or glass, containing electrodes 3 and having a constricted portion 4 between the electrodes. The

electrodes may be of the type generally used in,

capillary lamps, for example, they may consist of a central core of thorium within a spiral of tungsten wire or they may be formed by a tungsten coil activated by a suitable material such as barium or strontium carbonate. They may even be of the liquid type formed by pools of mercury, for example, if means are provided for cooling them to a sufiiciently low temperature. The diameter of the constricted portion is preferably about one millimeter but it may vary depending upon the total current passed through the lamp and the current density desired. The lamp I is surrounded by a water jacket 5 through which cooling fluid is passed for the purpose of lines app ar.

aiding the maintenance of low gaseous pressure within envelope 2 and at the same time preventing the softening of the envelope under the heat of the arc. If solid electrodes are used, a small amount of mercury or other suitable metal provides a source of metal vapor and is inserted in the envelope 2 in the usual manner during the Energy for operation of evacuation process. the lamp may be provided by the secondary winding 14 of the leakage reactance transformer l J which is energized by any suitable source of alvapor which the envelope would normally hold in vaporized state at the temperature of operation. This may be accomplished by limiting the quantity of material introduced into the envelope to such an extent that it is completely vaporized before the device attains its operating temperature. Of course, if an excess of mercury is used the pressure may still be kept low by vigorous artificial cooling but this procedure ,is much more diincult in practice.

I have found that by operating the described lamp at a gaseous pressure of about two atmospheres and with a total current of about five amperes (tube diameter equal to 1 mm.) a brilliant substantially white light is obtained. Under these conditions, the spark spectrum lines and the continuous radiation of the mercury spectrum begin to appear when the current reaches about 3 amperes and continuethereafter, as the current densityincreases, to add to the spectralcontent of the light. Generally speaking, therefore, at this pressure the desired efiect occurs when the current density in amperes becomes greater than about three amperes per square millimeter. If higher vapor pressures are used, it is necessary to go to higher current density before the spark Conversely, if. lower pressures are used, the same effect can be obtained by lower current densities. Generally speaking, the current required to produce the spark lines will be roughly proportional to the pressure or some low power thereof. However, since the lumen output per unit length of arc is greatly dependent upon pressure, a compromise must be struck between pressure and light output in order that a reasonably large output may be obtained without resorting to too high pressures. For example, I have found that at pressures below about one-half of one atmosphere, the light intensity becomes too low for general purposes. I have found the above pressure of two atmospheres to bea good compromise.

Under the conditions described, the vapor approaches 100 per cent ionization, i. e. almost all of the available molecules in the gas become ionized. At'such high degrees of ionization, the arc begins to lose its negative voltage'characteristic and begins to behave with positive characteristics. At

extremely high values .of current, the walls of the envelope can be made to incandesce and thus add to the light output.

A'lamp of the type shown may be constructed in the following manner. First, the water jacket 5 and constricted portion 4 are formed by sealin the ends 6 of a pair of concentric cylindrical .tubes having walls I and 8 and by providing fluid outlets 9 and Mn the usual manner. Next,'separate cup-shaped end-members l I may be formed by collapsing one end of a. short cylindrical tube about each of the lead wires I: of electrodes 3.

Finally, theopen ends of the-said cup shaped closed illustrates but one form of my invention and that many modifications within the scope of the appended claims may occur to those skilled in the art to which the invention appertalns. For example, many difierent tube configurations may be employed while anysuitable coolin -means, as cold'air blasts, may be used in place of the water cooling system shown. Further, other metallic vapors may be used, although in general the-specified current densities and pressures will be changed somewhat if the same results are to be obtained. All such modifications I aim to include within the scope of the appended claims.

What I claim asnew and desire to secure by Letters Patent of the lJn'ited'States is:

1-. The method ofop'e'rati'n a high-pressure metal-vapor arc' discharge lamp comprising a tube having a bore of capillary dimensions, which method comprises maintaining the vapor pressure at a value of the order of a few atmospheres,

'while raising the current density to a value in excess of about three amperes per square millimeter ofthe--cross-section ofthe tube bore so that the arc stream completely fills the bore of, the tube and spark spectrum .lines appear in the spectrum of the metal vapor.

2. The method of operating a high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of capillary dimensions, which method comprises maintaining the vapor pressure at a value of the order of a few atmospheres, while raising the current density to a value in excess of about three amperes per square millimeter of the cross-section of the tube bore at which the arc begins to lose its negative voltage characteristic and begins to behave with positive characteristics and spark spectrum lines appear in the spectrum of the metal vapor.

3. The method of operating a high-pressure metal-Vapor arc discharge lamp comprising a tube having a bore of capillary dimensions, which method comprises maintaining the vapor pressure at a value of the order of a few atmospheres by limiting the metal which produces said vapor to an amount such that it is completely vaporized before the lamp attains its operating temperature and by also artificially cooling the lamp, while raising the current density to a value in excess of about three amperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fills the bore of the tube and spark spectrum lines appear in the spectrum of the metal vapor.

4. The method of'operating a high-pressure metal-vapor arc discharge lamp comprising a which the lamp may be inserted while raising the current density to a value in excess of about three amperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fills the bore of the tube and spark spectrum lines appear in the spectrum of the metal vapor.

5. A high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of capillary dimensions, a pair of electrodes and a quantity of vaporizable metal for supporting a discharge through said tube, means for maintaining the vapor pressure at a value of a few atmospheres, and a source of electrical energy for supplying a current density in excess of about three amperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fills the bore of the tube and spark spectrum lines appear in the spectrum of the metal vapor.

6. A,high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of capillary dimensions, a pair of electrodes and a quantity of vaporizable metal for supporting a discharge through said tube, means for maintaining the vapor pressure at a value of the order of two atmospheres, and a source of electrical energy for supplying a current density in excess of about threeamperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fills the bore of the tube and spark spectrum lines appear in the spectrum of the metal vapor.

7. A high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of the order of a millimeter in diameter, a pair of electrodes and a quantity of vaporizable metal for supporting a discharge through said tube, means for maintaining the vapor pressure at a value of the order of two atmospheres, and a source of electrical energy for supplying a current density in excess of about three amperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fllls the bore of. the tube and spark spectrum lines appear in the spectrum of the metal vapor.

8. A high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of capillary dimensions, a pair of electrodes and a quantity of vaporizable metal for supporting a discharge through said tube, the said vaporizable metal being limited in amount so that it is all vaporized before the lamp attains its operating temperature, cooling means for maintaining the vapor pressure at a value of a few atmospheres, and a source'of electrical energy for supplying a current density in excess of about three amperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fills the bore of the tube and spark spectrum lines appear in the spectrum of the metal vapor.

9. A high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of capillary dimensions, a pair of electrodes and a quantity of vaporizable metal for supportinga discharge through said tube, the said vaporizable metal being limited in amount so that it is all vaporized before the lamp attains'its operating temperature, cooling means for maintaining the vapor pressure at a value of theorder of two atmospheres, and. a source of electrical energy for supplying a current density in excess of about three amperes per square millimeter of the cross-section of the tube bore so that the arc stream completely fills the bore of the tube and spark spectrum lines appear in the spectrum of the metal vapor.

10. A high-pressure metal-vapor arc discharge lamp comprising a tube having a bore of the order of a millimeter in diameter, a pair otelectrodes and a quantity of vaporizable metal for I meter of the cross-section of the tube bore so that the arc stream completely fills the bore of g the tube and spark spectrum lines appear in the spectrum of the metal vapor.

EUGENE mhluwl- 

